What is PCB impedance control?
PCB impedance control refers to controlling the impedance of the wiring, which is also known as controlled impedance. Controlled impedance is the characteristic impedance of a transmission line formed by the PCB routing and its related reference planes. When high-frequency signals propagate on PCB transmission lines, they are correlated. Controlled impedance is crucial for solving signal integrity issues, which involve undistorted propagation of signals.
The impedance of a circuit is determined by the physical size and dielectric material of the PCB. It is measured in ohms (Ω). The types of PCB transmission lines that require impedance control include single ended microstrip, single ended stripline, microstrip differential pair, stripline differential pair, embedded microstrip, and coplanar (single ended and differential).
Why control PCB impedance?
Usually, it is necessary to control the impedance of PCBs used for high-speed digital applications, such as radio frequency communication, telecommunications, calculations using signal frequencies above 100MHz, high-speed signal processing, and high-quality analog videos (such as DDR, HDMI, Gigabit Ethernet), ETC. The PCB in this type of PCBA needs to be impedance tested.
At high frequencies, the signal routing on the PCB is like a transmission line, with impedance at every point on the signal routing trajectory. If the impedance changes from one point to the next, there will be a signal reflection, and its amplitude will depend on the difference between the two impedances. The greater the difference, the greater the reflection. This reflection will propagate in the opposite direction of the signal, which means that the reflected signal will be superimposed on the main signal, resulting in distortion of the original signal.
The signal that was originally intended to be sent from the sender will change once it reaches the receiver. The distortion may be significant, to the extent that the signal may not perform the required function. Therefore, in order to achieve undistorted signal transmission, the PCB signal routing must have a unified controlled impedance to minimize signal distortion caused by reflection.
The uniform transmission line on the PCB has a determined routing width and height, and is at the same distance from the return path conductor, usually a plane with a certain distance from the signal routing.
Factors affecting PCB impedance control?
The factors that affect PCB impedance tolerance include the resin content percentage of the material, the Dk value of the resin, and the type of glass cloth used, as well as other physical PCB tolerances, such as the wire height and width at the top and bottom of the wiring.
When the PCB to be manufactured has the correct pattern size and position within a certain tolerance range. It is necessary to ensure that the manufacturer provides the correct dimensions, positions, and tolerances for etching features. Otherwise, each PCB board will be different, making debugging and performance related issues very difficult.
